Light sources



Sept 11, 1962 P. D. JOHNSON 3,054,013

LIGHT SOURCES Filed oct. 12, 1960 x E k @2.2. c h 0 '0? 1- 55a /a `J i f l l WAVL-/VG'TH 7'1"/ .3. g ,fj

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6J/W V @76 /s Attorney United States Patent O 3,954,013 LISHT SQURCES Peter i). dohnson, Schenectady, NSY., assigner to General Electric Company, a corporation of New York Filed Get. l2, i960, Ser. No. 62,267 i7 Ciaims. (all. .M3-19S) This invention relates to a new and improved light source of the type utilizing luminescent materials and in particular to such a light source utilizing luminescent single crystals.

As used throughout the specication and in the appended claims the term single crystal refers to a large, discrete body of material which is optically and crystallographically homogeneous. The single crystal referred to herein may be further defined for purposes of this invention as one having a high degree of crystal perfection.

For certain applications in the practice of this invention it is preferable to utilize the luminescent single crystals as thin slices or pieces of other geometric configuration. Such single crystal pieces may be obtained, for example, by cutting from a lange single crystal boule or ingot grown by pulling from a melt in known manner. For other applications, however, it is pereferable to utilize either an entire single crystal boule or ingot as grown or suitable lengths cut therefrom. The term single crystal, therefore, refers to an entire single crystal boule or ingot as grown as well as pieces of suitable geometric configuration cut therefrom. While single crystal ingots having a diameter as small as .3 millimeter may be utilized in the practice of this invention larger single crystals are preferably employed, as for example, single crystal ingots having a diameter of at least one centimeter and a length of at least five centimeters.

The above defined single crystals, therefore, are to be particularly distinguished from microcrystals which comprise conventional powdered luminescent materials. Such microcrystals may have any irregular geometry, may be fused masses which include a plurality of unoriented crystals and may have dimensions from one to forty microns.

While this invention is subject to a wide range of applications, it is suited for use in general purpose lighting and will be described particularly in that connection.

Two of the most widely used methods of producing light for general lighting purposes are exemplified by the incandescent lamp and the fluorescent lamp. In the conventional fluorescent lamp, ultraviolet radiation produced by a mercury discharge within the lamp is converted to visible radiation by a coating of a luminescent material on the -walls of the lamp. This coating is ordinarily made up of a luminescent material of microcrystalline powder mixed with an appropriate binder and applied in a layer of suitable thickness to `the lamp walls. The lamp conventionally contains a filling of mercury and an inert or noble starting gas at low pressure. Electrical energy is thus converted in a gaseous atmosphere into an ultraviolet radiation rich in energy. The ultraviolet radiation so produced is then converted into longer-wave visible radiation by the coating of luminescent material.

In addition to converting the mercury discharge radiation into visible radiation, however, the microcrystalline phosphor powder coating causes light scattering in all directions. Due to this scattering effect there is a substantial amount of light directed toward the ends of the lamp. The light directed toward the ends of the lamp cannot be utilized and this loss reduces the overall efficiency of the lamp. For this and other reasons it has long been desired to obtain a more attractive method of utilizing luminescent materials for illumination.

It is an object of this invention, therefore, to provide luminescent materials which has greater efficiency than prior known light sources of this type which utilize powdered phosphors.

It is another object of this invention to provide a new and improved light source utilizing luminescent materials wherein the emission therefrom is polarized.

Briefly stated, in accordance with one aspect of this invention, a new and improved light source comprises a luminescent halophosphate single crystal and means in juxtaposition therewith for producing ultraviolet radiation. The ultraviolet radiation so produced is intercepted by the halophosphate single crystal and converted to visible radiation by excitation of the single crystal to luminescence in the visible spectrum. The luminescence of the single crystal is strongly polarized with the electric vector parallel to the crystal optic axis so that the radia-` tion of the light source so provided is polarized accordingly. In addition, the light source is most efficient since no light is lost in the ends and substantially all the ultraviolet radiation produced is converted to visible light.

The novel features which I Ibelieve to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof will best be understood by reference t0 the following description taken in conjunction with the accompanying drawing wherein like parts are indicated by the same reference numerals and in which:

FIG. 1 is a curve illustrating the relative emission intensity as a function of wavelength fora typical luminescent halophosphate single crystal suitable for use in the practice of this invention,

FIG. 2 is a curve illustrating the polarization of emission of such luminescent halophosphate single crystals and light sources of this invention,

FIG. 3 is a diagrammatic view partly in section of one light source embodying this invention,

FIG. 4 is a curve showing diagrammatically the emission distribution from a luminescent single crystal or a light source embodying this invention,

FIGS. 5 and 6 are diagrammatic sectional views of other light lsources embodying this invention,

FIG. 6a is an enlarged view of one type of supporting and mounting means utilized in the embodiment of FIG. 6 and,

FIG. 7 is a digrammatic sectional view of still another light source embodying this invention.

Various alkaline earth halophosphate luminescent materials are known in the art in the form microcrystalline powders and have been utilized in conventional fluorescent lamps. Such powdered halophosphate phosphors are described and claimed in U.S. Patents Nos. 2,488,733 and 2,664,401, both of which are assigned to the assignee of the present invention.

In general, halophosphates are compounds having the structure of the natural mineral apatite. These compounds may be represented by the general formula where X represents a halOgen or a mixture of halogens and M and M represent either the same or different bivalent metals or mixtures of such metals. The halophosphate phosphor powders described in the above re- -ferred to patents are very useful luminescent materials and have been utilized successfully in the construction of the conventional fluorescent lamps.

I have found that various halophosphates having many of the same properties of color and luminescent efficiency as the known powdered materials can be prepared as relatively large, light transmissive single crystals by employing suitably modified crystal growing methods. For example, suitable luminescent halophosphate single crystellsV for use in the practice of this invention, have been prepared by pulling from a melt of the material which is maintained at a temperature just above its freezing temperature. Various methods of growing crystals from a melt are known to the art further details of which may be had, Aif desired, by reference to the text entitled Crystal Growth by H. E. Buckley, published in 1951 by John Wiley and Sons, Inc., New York,

Although the single crystals of `the halophospha-tes of -this'invention have certain propertieswhich are'similar to those of known yhalophosphate phosphor powders such as, for example, structure of the host lattice, color and luminescent etliciency, it is to be observed that these single crystals differ in composition from the powdered materials and in addition exhibit certain new and useful properties. I have found, for example, that halophosphates activated with from about .001 to .l mole percent of a metal selected from the group consisting of antimony, lead, tin, thallium and copper and up to about .l mole percent of manganese as a supplemental activator, are luminescent over a range of color from blue through white to the pink. In addition, I have found that the luminescence of such crystals is strongly polarized having the electric vector substantially parallel to the crystallographic c-axis. This c-axis is referred to throughout the specilication and in the appended claims as the crystal optic axis.

I have further found that the luminescent single crystals so produced may be utilized to provide a new and improved light source having improved efficiency, especially as compared to light sources of the type utilizing powdered luminescent materials, and one ywhich provides polarization of emission.

Suitable single crystals of halophosphate for use in the practice of this invention have been prepared, using suitably modied apparatus, by pulling from a melt of activated halophosphate. Briefly, `such method involves pulling a single crystal from a melt maintained at a temperature just barely above its freezing temperature. Such pulling is often accompanied by rotation as well. To prevent chemical decomposition of the melt it is customto continually hush the surface of the melt wiah a nonreactive gas.

The modified apparatus involved the use of crucibles formed of a platinum 20 percent rhodium alloy. This alloy was yfound to be chemically inert with respect to the halophosphates to be melted. Such crucibles were found to be very satisfactory for this purpose as were those formed of iridium.

An atmosphere of air, carbon dioxide, nitrogen or a mixture of carbon dioxide and nitrogen has been found 'satisfactory for flushing the surface of the melt. Reducing atmospheres attack the halophosphates and more strongly oxidizing atmospheres than air tend to cause excessive oxidization of ithe supplemental manganese activator. The higher concentrations of a desired activator, such as for example, antimony, may be introduced by providing a partial pressure of that metal during the crystal growing process. This is often desirable particularly with the more volatile activators.

The activated halophosphate single crystal may be successfully pulled from .the melt with a rod formed of a. platinum 20% rhodium alloy. Luminescent halophasphate single crystals having a diameter of at least one centimeter are readily provided. Alternatively, single crystals of unactivated halophosphate may be prepared according to the above described method with suitable activators introduced thereafter by diffusion in a manner known to the art. For example, the unactivated single crystal may be sealed in an atmosphere containing a -partial pressure of the desired activator or activators and heated to a temperature and for a time sufficient to cause the activators to be diffused into the crystal.

The halophosphates suitable for purposes of this invention are of the structure of the natural mineral apatite and copper.

wherein at least half of the bivalent atoms are those of the alkaline earth metals, magnesium, calcium, strontium and barium or mixtures thereof while at -least half of the halogen atoms are of the group consisting of chlorine, fluorine and mixtures thereof. This halophosphate is activated with from .001 to .l mole percent of a metal selected from the group consisting of antimony, lead, tin, thallium In addition, besides the primary activators listed in the group above, the halophosphate may also contain manganese as a supplemental activator. The addition of a suitable quantity of manganese makes possible a wide range of color. For example, when the primary activator is antimony the color of the emission ranges from the blue with no manganese to pink with a concentration of manganese of about .1 mole percent and a concentration of antimony of about .0l mole percent. By combination of manganese with the primary activator to achieve a suitable balance of activators it is easily possible to produce approximately a white light when desired. The relative emission of such a luminescent single crystal, for example, is illustrated by the curve of FIG. l showing the relative emission intensity as a function of wavelength. The region indicated at A represents the emission peak due to the antirnony activator While the re-gion indicated at B represents the emission peak due to the manganese activator. The single crystals of this invention may be excited to luminescence -by ultraviolet radiation, cathode rays, X-rays or a combination of such means.

The activated single crystal halophosphates so prepared are found to exhibit a strongly polarized luminescence the electric vector of which is substantially parallel to the crystal optic axis. This polarization of emission is illustrated by the graph of FIG. 2 showing intensity as a function of the angle, 0, between the electric vector, E, and the crystal optic axis.

This invention may be embodied in a fluorescent lamp of the well-known kind as illustrated in FIG. 3. The lamp of FIG. 3 comprises a vitreous enclosing envelope 1 having therein an ionizable medium such as a quantity of mercury indicated by the globule 2. In addition to the mercury there is also provided a lling of an inert or noble starting gas 3 at low pressure as is conventional.

Electrodes 4 and 5 at opposite ends ofthe envelope are supported on lead Wires 6 sealed through the ends of the lamp. At least one of the electrodes is of the thermionic type. It is to be understood by those skilled in the art that the use of two pins at each end of the lamp is for illustrative purposes only, the form and arrangement of such terminals depending upon the electrodes employed and the operating circuits connected thereto. Conduction of electric current through the ionizable medium produces the ultraviolet exciting radiation.

A plurality of single crystals 8 having their optic axes oriented parallel to each other are suitably secured to the walls of the envelope 2. To this end a layer 9 of a light transmissive bonding material, such as a low melting glass for example, may be utilized and the single crystals suitably fused thereto. Other examples of suitable bonding materials are epoxy resins and hydroquinone isophthalate-terephthalate resins. Preferably, the bonding material is chosen to have an index of refraction approximately the same as those of the single crystals.

Although for purposes of illustration the single crystals 8 are shown secured in oriented, contiguous relationship to the inside walls of envelope 2 it is to be understood that the crystals may be similarly secured to the outside of the envelope, provided, however, that the envelope is of a material pervious to ultraviolet radiation. In this respect it is only required that the crystals be disposed with respect to the envelope 2 so that the ultraviolet radiation is intercepted by the crystals causing them to be excited to luminescence and effectively converting this short-wave radiation into the longer-wave visible radiation.

The single crystals 8 are of a material consisting esin the halogen is of the group consisting of chlorine, uorine and mixtures thereof activated `with from about .G01 to .l mole percent of a metal selected from the group consisting of antimony, lead, tin, thallju-m and copper and from about G to .l mole percent of manganese as a supplemental activator. The ultraviolet radiation of the mercury discharge is incident on the single crystals 8 and causes emission therefrom in the visible spectrum. As described, hereinbefore, the color of this emission may range from blue to white through pink depending upon the relative concentrations of the respective primary activator and supplemental manganese activator.

As shown by the graph in FIG. 2 the emission of each of the single crystals is strongly polarized with the electric vector being substantially parallel to the crystal optic axis. Each of the crystals may be suitably disposed on the walls of the lamp with its optic axis oriented parallel with that of each of the other crystals. In this way the visible radiation produced by the lamp is polarized in a direction parallel to the optic axes of the oriented crystals. F or example, where the optic axes of all crystals are parallel with respect to each other and parallel with the major axis of the lamp, the visible radiation of the lamp is polarized in a direction parallel to its major axis. In addition, there is substantially no emission from the crystals in a direction parallel to their optic axes and, therefore, substantially no light is lost in the ends of theV lamp. The curve of FIG. 4 showing the distribution of light from a lamp or excited single crystal with respect to the major or optic axis respectively, illustrates this characteristic. Since in ordinary uorescent lamps `light is lost in the ends, the lamp provided by this invention has `greater efficiency than prior known lamps and particularly when compared to lamps of this type utilizing known microcrystalline powdered luminescent materials wherein the emission is scattered in all directions.

In FIG. 5 there is shown a `diagrammatic sectional view of another embodiment of the lamp of lthe present invention. The lamp, generally designated `at 10, includes a single crystal 11 of suitably activated halophosphate alkaline earth metal phosphor prepared, for example, as described in detail hereinbefore. Single crystal 11 has a bore 12 therein parallel with its optic axis. End caps 13 and 14 are provided and sealed to opposite ends of crystal 11 and form therewith an enclosing envelope. End caps 13 and 14 may be formed of metal or other suitable material and may conveniently be sealed to the ends of crystal ,11 with a suitable bonding material such as, for example, epoxy resin or hydroquinone iso-phthalateterephthalate resins.

The envelope so formed may contain, as before, an ionizable medium such as mercury and an inert or noble starting as at low pressure as is conventional. The mercury and inert gas lling are shown by reference numerals 15 and 16 respectively. Conduction of current through the ionizable medium produces the desired ultraviolet radiation for exciting single crystal 11 to luminescence. Alternatively, the ultraviolet radiation may be provided from a separate self-contained source.

-The ultraviolet radiation produced within bore 12 is incident on the single crystal 11 causing the crystal to emit visible radiation. As described hereinbefore and illustrated by the curves of FIGS. 2 and 4, the luminescence of Ithe crystal is strongly polarized. For example, the visible radiation of the lamp of FIG. 5 is polarized in a direction parallel to the crystal optic axis. Again, there is substantially no radiation from the ends of the crystal or light scattering to lower the eiciency of the lamp.

It -is known Vthat strong sources of energy for ultraviolet light exist and these can be employed to advantage in the light `source according to this invention. Many of these strong ultraviolet sources of energy produce in addition a substantialamount of visible radiation. Since the luminescent single crystals of this invention are light transmissive, full utilization of these strong ultraviolet sources may be made to produce Visible light. For example, the

visible portion of the radiation of the ultraviolet sourcev is freely transmitted through the single crystals of halophosphate phosphor u-tilized in this invention while at the same time the ultraviolet portion of the radiation is eiliciently converted thereby to visible radiation. The resulting light source is extremely eicient and in addition again provides a source which is polarized. This can occur, for example, in the light source shown particularly by the embodiments of FIGS. 6 and 7. This is in contrast to the conventional fluorescent lamps for example, which are subject to some loss in the microcrystalline powder phosphor layer.

In FIG. 6 an enclosing envelope is formed of a single crystal `of luminescent halophosphate having a bore parallel to its opt-ic axis as described in detail for the embodiment of FIG. 5. A self-contained ultraviolet source 117 extends through bore 12 of single crystal 11. Ultraviolet source 17 may be, rfor example, a small, high energy ultraviolet lamp of well-known type and is mounted and supported within bore 12 as by means of annular supponts 18 and 19. Alternatively, ultraviolet source 17 may be mounted in bore 12 by suitable caps fastened to the ends of single crystal 11 or other suitable means known to the art. Preferably the mounting and supporting means for ultraviolet source 17 has provision for allowing the vliow of ambient air through bore 12 as shown, for example, by FIG. 6a. When ultraviolet source 1 7 is energized the radiation produced thereby is incident o n crystal 11 throughout the length of bore 12. Crystal 11 is thus excited to luminescence and produces visible radiation. Since ultraviolet source 17 is self-contained and there is no ionizable medium utilized within bore 12, end supports 18 and 19 need not be sealed in a gas tight manner to crystal 11 and may be mounted therein in any convenient manner such as, for example, a friction tight fit or if end caps are utilized such caps may be fastened -to single crystal 11 by means of a plurality of screws.

Another embodiment utilizing a strong ultraviolet source-is shown in FIG. 7. In FIG. 7, a source of ultraviolet radiation 20 is arranged in the focal point of an interiorly specular reector 21. The open end of reflector 21, which is conveniently symmetrical in relation to its axis of rotation, is closed with a transparent face plate 22. Transparent face plate 22 may be, for example, of glass and has secured thereto with a suitable light transmissive bonding material such as a low melting glass, a plurality of oriented single crystals 8 of a material consisting essentially of a luminescent halophosphate of alkaline earth metal such as described in detail hereinbeyfore. Ultraviolet source 20 may be an integral part of reector 21 wherein the lamp envelope is also the envelope for the ultraviolet source. Alternatively, ultraviolet source 2t) may be a separate self-contained unit mounted Within the reflector 21 both well-known to the art.

As shown by the foregoing detailed description, the present invention may be embodied in lamps of wellknown -type as well as lamps of a new type which utilizes the activated single crystal halophosphate itself as the lamp envelope. The structures of the several embodiments shown are not to be construed as limiting this yinvention. The present invention is applicable to many and varied lamp congurations only a few of which have been described in detail as specific embodiments. In addition, because of its polarized property, the new and improved Ilight source of this invention may be employed v 7 in a more ecient manner than heretofore and in addition has provided a light source having polarization of emission.

While only certain preferred features of the present invention have been shown by way of illustration, many modifications and changes will occur to those skilled in fthe art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes 'as fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent -by the United States is:

1. A ylight source comprising: a luminescent halophosphate single crystal and means in juxtaposition to said single crystal for producing ultraviolet radiation, said radiation being intercepted by said single crystal `and converted to polarized visible radiation by excitation of said single crystal to luminescence in the visible spectrum, said visible radiation being polarized in a direction parallel with the optic axis of said crystal so that the output of visible radiation therefrom has an angular distribution which ranges from substantially zero in the direction parallel with said optic axis to a maximum in the direction perpendicular to said optic axis.

2. A light source comprising: a single crystal of a halophosphate of alkaline earth metal wherein the halogen is of the group consisting of chlorine, uorine land mixtures thereof -activated with from about .001 to .1 mole percent of a metal selected from the group consisting of antimony, lead, tin, thallium and copper and up to .1 mole percent of manganese as a supplemental activator; and means in juxtaposition to said single crystal for producing ultraviolet radiation, said radiation bein-g intercepted fby said single crystal and converted to polarized visible radiation by excitation of said single crystal to luminescence in the visible spectrum, said visible radiation being polarized in a direction parallel with the optic axis of said crystal so that the output of visible radiation therefrom has an angular distribution which ranges from substantially zero in a direction parallel with said optic axis to a maximum in the direction perpendicular to said optic taxis.

3. A light source comprising: an envelope having at least one wall pervious to light, said wall comprising a single crystal consisting essentially of a halophosphate of alkaline earth metal wherein the halogen is of the group consisting of chlorine, iiuorine and mixtures thereof activated with from about .,001 to .l mole percent of a metal selected from the group consisting of antimony, lead, tin, thallium and copper and up to about .1 mole percent of manganese as a supplemental activator, said single crystal being disposed so that the optic axis thereof is parallel with -the major axis of said envelope; and means producing ultraviolet radiation within said envelope for exciting said single crystal to luminescence and causing visible radiation to Ibe produced therefrom which is polarized in a direction parallel to the optic axis of said crystal so that the output of visible radiation therefrom has fan angular distribution which ranges from substantially zero in the direction parallel with said opti-c axis to a maximum in the direction perpendicular to said optic axis.

4. The light source of claim 3 whereinthe ultraviolet radiation is produced by a mercury discharge established withinsaid envelope.

5. The light source of claim 3 wherein the ultraviolet radiation is produced by a self-contained source disposed within said envelope.

6. The light source of claim 3 wherein the ultraviolet radiation is produced by current conduction through an ionizable medium of mercury and a noble gas at low pressure within said envelope.

7. A light source comprising: a light transmissive evacuable envelope; means for establishing a mercury discharge Within said envelope; a plurality of single crystals secured in oriented, contiguous relationship to the surface of said envelope with the optic axes of said crystals parallel with the major axis of said envelope and arranged so as to cover the surface of said envelope and intercept the radiation of said mercury discharge, said crystals consisting essentially of a halophosphate of alkaline earth metal wherein the halogen is of the group consisting of chlorine, iiuorine Aand mixtures thereof activ-ated with from 0.001 to 0.1 mole percent of a metal selected from the group consisting of antimony, lead, tin, thalliurn and copper land from 0 to .l mole percent of manganese as a supplemental activator, the radiation of said mercury discharge being converted to polarized visible radiation by excitation of said single crystals to luminescence in the visible spectrum, said visible radiation being polarized in a direction parallel with the optic axes of said crystals so that the output of visi-ble radiation from said light source has an angular distribution which ranges from substantially zero in the direction parallel with the major axis of said envelope to a maximum in the direction perpendicular to the major axis of said envelope.

8. The light source of claim 7 wherein the single crystals are secured to said envelope with alight, transmissive material having an index of refraction substantially the same as that of said crystals.

9. The light source of claim 7 wherein the material for securing the single crystals to said envelope is a low melting glass.

10. A light source comprising: a single crystal of a halophosphate of alkaline earth metal wherein the halogen is of the group consisting lof chlorine, iuorine and mixtures thereof activated with `from 0.001 to 0.1 mole percent of .a metal selected from the group consisting of antimony, lead, tin, thallium and -copper and up to about 0.1 mole percent of manganese as a supplemental activator, said crystal having a bore therein parallel with its optic axis; a pair of end caps sealed to the ends of said crystal and forming therewith an evacuable envelope; and means for producing ultraviolet radiation wi-thin said bore so that said single crystal is excited to luminescence and produces visible radiation which is polarized in a direction parallel with the optic axis of said crystal.

11. A light source comprising: a single crystal having a central bore therein parallel to its optic axis, said crystal consisting essentially of a halophosph-ate of alkaline earth metal wherein the halogen is of the group consisting of chlorine, iiuorine and mixtures thereof activated with from 0.001 to 0.1 mole percent of a metal selected from the group consisting of antimony, lead, tin, thallium and copper and up to 0.1 mole percent of manganese as a supplemental activator; 4and means within said bore for exciting said crystal to luminescence to cause said crystal to emit visible radiation in response thereto, said visible radi-ation l*being polarized in a direction parallel with the Opto axis of said crystal so that there is substantially no visible radiation in a vdirection parallel with said optic axis.

12. A light source comprising: an enclosing envelope, said envelope being a single crystal having Ia bore therein parallel to its optic axis and consisting essentially of a halophosphate of alkaline earth metal wherein the halogen is of the group consisting of chlorine, iiuonne and mixtures thereof activated with from about .001 to .1 mole percent of a metal selected from the group consisting of antimony, lead, tin, thallium and copper and up -to abouti mole percent of manganese as a supplemental activator; and means Within said envelope providing a source of ultraviolet radiation, said ultraviolet radiation exciting said single crystal envelope to luminescence and causing visible radiation polarized in a direction parallel with the optic axis of sai-d crystal to tbe produced therefrom.

13. A fluoroescent lamp comprising: an enclosing envelope having therein a pair of electrode and an ionizable medium comprising mercury vapor and an inert gas; and

a plurality of single crystals having their optic axes oriented parallel with each other and with the major axis of said envelope disposed on the inside surface of said envelope for transforming the ultraviolet radiation produced `by conduction of current through said ionizable medium to visible radiation polarized in a direction parallel with the major axis of said envelope, said crystals consisting essentially of a halophosphate of alkaline earth metal wherein the halogen is of the group consisting of chlorine, iluorine and mixtures thereof activated with from about .O01 to .1 mole percent of a metal selected from the group consisting of antimony, lead, tin, thallium and copper and up to about .1 mole epercent of manganese as a supplemental activator.

14. A light source comprising: an envelope including a Wall pervious to light; a plurality of single crystals having their optic axes oriented parallel with each other disposed in contiguous relationship over the surface of said wall, said crystals consisting essentially of a halophosphate of alkaline earth met-al wherein the halogen is of the group consisting of chlorine, fluorine and mixtures thereof activated with from about .O01 to .1 mole percent of a metal selected from the group consisting of antimony, lead, tin, thallium and copper and up to about .1 mole percent of manganese as a supplement activator; and means providing a source of ultraviolet radiation Within said envelope for exciting said plurality of single crystals to luminescence and causing visible radiation to be produced which is polarized in a direction parallel with the optic axes of said crystals.

15. A light source comprising: a luminescent halophosphate single crystal having a central bore therein parallel with its optic axis; means for sealing the open ends of said central bore to form yan evacuable envelope from said single crystal; and means Within said bore for producing ultraviolet radiation, said radiation being intercepted by said single crystal and converted to visible radiation which is polarized in a direction parallel with the optic axis of said single crystal envelope.

16. A light source comprising: a luminescent halophosphate single crystal having a central bore therein parallel with its optic axis; and a source of ultraviolet radiation disposed within the central bore of said single crystal, said ultraviolet radiation being intercepted by said single crystal and converted to visible radiation which is polarized in a direction parallel with said optic axis.

17. A light source comprising: a luminescent halophosphate single crystal having a central bore therein parallel with its optic axis; a self-contained source of ultraviolet radiation; means for mounting said source of ultraviolet radiation Within the central bore of said single crystals, said means allowing for the ow of ambient air through said central bore, said ultraviolet radiation being intercepted -by the surrounding single crystal and converted to visible radiation which is polarized in a direction parallel with the optic axis of said crystal.

References Cited in the le of this patent UNITED STATES PATENTS 

